The present invention relates to a process for the manufacture of low-density components, having a polymer or metal matrix substrate, ennobled with a ceramics and/or metal-ceramics coating, capable of improving the performances of the components in all the situations requiring high surface strength. The process of the invention allows the application on said substrates of protective hard coatings, like, e.g., the carbide-, boride-, nitride-based ceramic ones, capable of remarkably improving the surface strength of the underlying low-density structural material.
As it is known, for application in the industrial, aeronautical and space fields there subsists a need for the availability of compounds capable of competing with the high performances of the steels, yet exhibiting lower specific weight.
EP-A-0,164,617, DE 35 27 912 A, U.S. Pat. No. 5,521,015 disclose process of hot spraying deposition of a coating having a strength greater than that of the respective low density substrate.
The present invention allows to comply with the above-mentioned need, further providing other advantages that will hereinafter be highlighted.
In fact, the present invention relates to a process for the manufacture of low density components, having a polymer or metal matrix substrate, and ceramics and/or metal-ceramics coating, in which the low density substrate to be coated is subjected to the following steps:
machining the surface in order to generate residual compressive stress in the outer layers;
thermal stabilising at a temperature lower 350xc2x0 C.;
depositing onto the outer surface, with hot spraying techniques at a temperature ranging from 70xc2x0 to 350xc2x0 C., of a coating layer in a ceramics and/or metal-ceramics material with a surface strength higher than that of the component to be coated; and
finishing the surface of the coating layer by a finishing treatment.
The surface machining, in order to generate residual compressive stress in the outer layers of the component to be coated consists of a treatment selected from the group consisting of peening and/or sandblasting and/or combinations thereof.
The finishing treatment of the surface of the coating layer comprises of a machining selected from the group consisting of grinding, polishing, tumbling, rumbling and combinations thereof.
The hot spraying techniques are selected from the group comprising high velocity hot spraying (HVOF, High Velocity Oxy-Fuel), plasma spraying (VPS-Vacuum Plasma Spraying, CAPS-Controlled Atmosphere Plasma Spraying, APS, HPPS), Flame Spraying (FS), Plasma Transferred Arc (PTA), Arc Spraying (AS), and combinations thereof.
The hot sprayed coating layer has a thickness comprised in the range from 100 to 4200 xcexcm, preferably from 100 to 500 xcexcm.
The coating layer is selected from the group consisting of WC-M, CrC-M, TiC-M, BN-M, SiC-M, wherein M is the metal matrix selected from the group consisting of Ni, Co, NiCr, NiCrFeBSi, NiCrCuMoWB.
It has been observed that satisfactory results are obtained in the present invention adopting low density materials exhibiting an E/p (Modulus of elasticity/specific weight) value of the same order of that of the reference 17-4PH steel (E/p=25 GPa/kg/dm3).
Accordingly, light metals, like aluminium and titanium, Ti/Al alloys, metal matrix composites thereof and polymer matrix composites (usually made of fibres immersed in a polymer matrix) were found to be suitable for use as substrates in the present invention.
Concerning the metal matrix composites, satisfactory results were obtained with compounds made of an aluminium matrix charged with a charge percent of about 10-20% titanium carbide (yielding higher E and coefficient of thermal expansion xcex1 with respect to the pure aluminium) and a composite made of titanium charged with 10-20% titanium carbide. The E/p ratio for these composites is 28.6 and 28.2 GPa/kg/dm3, respectively. In order to compare the characteristics of these materials, it has to be pointed out that the AA7075 aluminium alloy and the T6Al4V titanium alloy exhibit E/p values of 26.7 and 24.2, respectively (see also the comparison reported in Table 1).
Concerning the composite materials, their properties depend on the matrix and fibrefill selection.
In this respect, carbon fibres which have moduli of elasticity ranging from 160 (low modulus) to 725 (very high modulus) are of special interest. Highly promising are, e.g., the carbon-carbon, composites made of carbon fibres in a carbon matrix, having a modulus of elasticity ranging from 125 to 220 GPa. These materials have an 1.3-1.6 kg/dm3 density, thereby yieldingxe2x89xa778 (GPa/kg/dm3) E/p values.
Other highly promising fibrefill are the boron fibres, having a modulus of elasticity of about 400 GPa, though being accordingly more expensive, with respect, e.g., to the carbon fibres (approximately 2-fold with respect to a High Modulus).
Another crucial aspect that needs considering re the fibrefill concerns the glass fibres, having moduli of elasticity ranging from 69 to 86 GPa with 2.4-2.6 kg/dm3 densities, and hence seemingly not useful in several industrial, aeronautical and space fields.
The selection of the composite material matrix deserves a much ampler account. In this respect, it has to be pointed out that satisfactory results were attained with the polyetheretherketone, commercially known as PEEK, and with an epoxy resin.
The characteristics of these two resins are reported in Table 2 and in Table 3, respectively.
In the case of polymer matrix composite materials, the most promising hot spraying coating techniques are the Plasma Spraying (PS) and the High Velocity Oxy-Fuel (HVOF), as these exhibit a low thermomechanical load with respect to other hot spraying technologies. Concerning instead the metal matrix composites to be coated, the spraying technologies have a quite small thermomechanical impact thereon.
The invention is not limited to the process for the manufacture, also extending to the low-density, high surface strength, coated components thus obtained.
So far, a general description of the present invention has been provided. With the aid of the single annexed FIGURE (FIG. 1) and of the examples hereinafter a more detailed description of specific embodiments, aimed at making better understood the objects, the features, the advantages and the operation modes thereof, will be provided.
FIG. 1 is a perspective view of a recirculating ball unit, made of a raceway P, coated with an embodiment of the process according to the present invention, and a ball slide S.